Isothiazoles with an alkyl substituent in the 3-position, a cyano group in the 4-position, and a grouping --NHCONR.sup.3 R.sup.4 in the 5-position, R.sup.3 and R.sup.4 each being methyl or ethyl, possess herbicidal properties against dicotyledonous and monocotyledonous weeds.